This invention pertains to radio-wave receiving apparatus, and particularly to apparatus for canceling radio-wave interference.
The so-called "Collins system" utilizes a second antenna tuned to a frequency away from the band of frequencies sought to be received to intercept bursts of interference that hopefully also affects reception within the wanted band of frequencies.
The radio-wave energy from the second antenna is detected to an audio frequency, typically, and then a value of direct current of corresponding amplitude is formed.
This direct current is fed into the intermediate frequency amplifier of the main receiver in such polarity as to reduce the amplification of the interference-experiencing intermediate frequency amplifier to substantially zero.
Unfortunately, the wanted signal as well as the interfering signal is thereby eliminated from the output of the main receiver for the duration of the burst of interference.
In the early 1960's a so-called "Range Extender" circuit was adapted from television practice for communications noise-cancelling. However, only one antenna was used and the phase-reversing noise-cancelling device was merely a shunt circuit in the single antenna lead-in.
Again, but for a different reason than that given above, the wanted signal as well as the interfering signal is eliminated.
The simple amplitude-limiter at any point in the main receiver to limit interference to approximately the maximum level of the desired signal, is also known. However, the removal of the interference is only partial and only for interference having an amplitude of greater than the desired signal.